Marine Glossary

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Lunar altitude

This is a method of finding longitude and correct time from the altitude of the moon. The process requires a moon bearing roughly east or west at twilight. The procedure is to take a series sights of two stars and the moon, alternating the sights. You need a watch, but it can have an unknown error in it. It is this error we are trying to discover.

After a careful sequence of at least 4 each are taken, plot them all on the same graph paper (Hs vs. WT). Then draw a line as best fit for each set of data, and then select a time and from that graphically determine the "simultaneous" heights of all 3 bodies. Where the time line crosses the sight line we assume is the height you would have measured if you took the sight at that particular time. You will not have actual sights at that time.

Then do normal sight reductions of all 3 sights. The two stars will intersect at your correct latitude, but the moon sight will differ from the intersection by an amount that depends on the watch error. Guess a new watch time and reduce the sights again and see if the moon sight is closer or farther from the intersection.

In this manner you can home in on the right time that makes the sights coincide. We have more specific instructions to expedite the process in the Emergency Navigation book. There is also an example worked out with the StarPilot in the StarPilot section of our website.

This method has an advantage over the lunar distance method in that no special sights nor procedures are necessary, although it is not quite as accurate and cannot be done on any random day.